Influence of Ethanol Parametrization on Diffusion Coefficients Using OPLS-AA Force Field.

Area of Science:

• Computational chemistry
• Physical chemistry
• Materials science

Background:

• Molecular dynamics (MD) simulations are vital for understanding liquid properties.
• The Optimized Potential for Liquid Simulations all-atom (OPLS-AA) force field is widely used.
• Accurate prediction of diffusion coefficients in ethanol requires precise force field parameters.

Purpose of the Study:

• To determine self-diffusion coefficients (D11) of ethanol and tracer diffusion coefficients (D12) of solutes in ethanol using MD simulations.
• To reoptimize the diameter of the ethanol oxygen atom (σOH) in the OPLS-AA force field to improve simulation accuracy.
• To assess the impact of the reoptimized σOH on the diffusivities of various solutes and ethanol itself.

Main Methods:

• Performed MD simulations using the OPLS-AA force field under various temperature and pressure conditions.
• Reoptimized the σOH parameter by comparing simulated diffusivities with experimental data for quercetin and gallic acid.
• Validated the new σOH value by calculating diffusion coefficients for ibuprofen and butan-1-ol, and ethanol self-diffusion.

Main Results:

• The original OPLS-AA σOH (0.312 nm) resulted in >25% deviation for protic solute diffusivities.
• Reoptimizing σOH to 0.306 nm reduced average absolute relative deviations to 3.71% (quercetin) and 4.59% (gallic acid).
• The adjusted σOH improved predictions for ibuprofen (1.55% AARD), butan-1-ol (4.81% AARD), and ethanol self-diffusion (3.51% AARD).

Conclusions:

• The reoptimized σOH parameter significantly enhances the accuracy of MD simulations for protic solute diffusion in ethanol.
• The original σOH remains suitable for non-polar solutes and for estimating equilibrium properties like enthalpy of vaporization and density.
• This work provides a refined force field parameter for accurate molecular dynamics studies of ethanol-based systems.